This project seeks to characterize specific antibodies that protect infants from symptomatic shigella infection. We previously have shown that the concentration of milk antibody to the group of shigella virulence plasmid antigens determines the symptom status of breastfed infants who become infected by Shigella. Our studies have not, however, demonstrated which specific antibody(ies) are responsible for protection. We hypothesize that milk secretory IgA (sIgA) to specific virulence plasmid antigens and to specific epitopes on those antigens is responsible for the protective effect of human milk. Seven Specific Aims are proposed to address this hypothesis. Specific Aim 1: Define which antibodies to invasion plasmid antigens are associated with protection from Shigella invasion in a HeLa cell model. Specific Aim 2: Determine whether sIgA directed against one or more invasion plasmid antigens is associated with symptom status during Shigella infection in breastfed infants. Specific Aim 3: Determine the epitopes of Shigella invasion plasmid antigens which are detected by human milk sIgA. Specific Aim 4: Determine which antibodies to specific epitopes are related to protection in human infants. Specific Aim 5: Purify epitope specific sIgA directed against Shigella invasion plasmid antigens. Specific Aim 6: Determine the role of epitope specific antibodies by evaluating the ability of affinity purified antibodies to prevent invasion of HeLa cells by Shigella flexneri, S. sonnei, S. boydii, S. dysenteriae and invasive E. coli. Specific Aim 7: Determine which amino acid residues of invasion related epitopes are necessary for sIgA binding. Understanding how human milk antibodies protect infants from symptomatic shigellosis will lead to new insights into both the role of human milk and into the pathophysiology of shigella infections. A practical benefit of these studies is that they will lay the basis for more rational vaccine strategies.